Everything You Need to Know about Narcissists, Psychopaths, and Abuse - click on this link: http://www.narcissistic-abuse.com/faq1.html
Having sex with a first-degree blood relative is like having sex with oneself. It is a Narcissistic act and like all acts Narcissistic, it involves the objectification of the partner. The incestuous Narcissist over-values and then devalues his sexual partner. He is devoid of empathy (cannot see the other's point of view or put himself in her shoes).
As siblings or progeny grow older, the narcissist begins to see their potential to be edifying, reliable and satisfactory Sources of Narcissistic Supply. His attitude, then, is completely transformed. The former threats have now become promising potentials. He cultivates those whom he trusts to be the most rewarding. He encourages them to idolise him, to adore him, to be awed by him, to admire his deeds and capabilities, to learn to blindly trust and obey him, in short to surrender to his charisma and to become submerged in his follies-de-grandeur.
It is at this stage that the risk of child abuse - from emotional incest and up to and including outright incest - is heightened. The narcissist is auto-erotic. He is the preferred object of his own sexual attraction. His siblings and his children share his genetic material. Molesting or having intercourse with them is as close as the narcissist gets to having sex with himself.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - click on this link to purchase the print book, or 16 e-books, or 3 DVDs with 16 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)